Regarding a sound effect device for a musical instrument, a signal processing apparatus designed to regenerate features of timbre of a vacuum tube type guitar amplifier, especially timbre changes occurring when an excessive input is applied, is disclosed in patent document 1. FIG. 18 shows a configuration of the signal processing apparatus. The signal processing apparatus shown in FIG. 18 includes Ig blocks 301 and 302, Ip/Eg blocks 305 and 306, a level detector 308, a multiplier 309, and adders 303, 304, 307 and 310.
The Ig blocks 301 and 302 simulate the characteristics of a filter connected to the grid of a vacuum tube type power amplifier according to the input signal. The output signals from the Ig blocks 301 and 302 are supplied to the Ip/Eg blocks 305 and 306. The Ip/Eg blocks 305 and 306 simulate a plate current which changes depending on changes in the grid voltage in the vacuum tube type power amplifier.
The Ig blocks 301 and 302 has cut off characteristic becoming higher as the input voltage increases. The level detector 308 detects the output level obtained from the adder 307 adding the output level of the Ip/Eg block 305 and the output level of the Ip/Eg block 306. The operation of the vacuum tube amplifier is simulated by increasing the bias of the Ip/Eg blocks 305 and 306 by using the multiplier 309 and the adders 310, 303 and 304, as the added output level becomes larger.
As discussed above, according to the patent document 1, the characteristics of the filter connected to the grid of the vacuum tube type power amplifier and the statistic characteristic of the plate current are simulated depending on the input signal, and the bias to be added to the filter characteristic and the input signal is changed depending on the output signal level. As a result, the characteristic of the vacuum tube amplifier can be simulated, and the timbre of a vacuum tube type guitar amplifier, especially timbre changes occurring when an excessive input is applied can be regenerated.    Patent document 1: JP-A-07-86840